5...a6 is the characteristic move of the Najdorf Variation of the Sicilian Defence, the most popular variation of the entire Sicilian Defence. The variation is named after the Polish-Argentinian Grandmaster Miguel Najdorf. In one sense, 5...a6 is a waiting move; White has no immediate threats, so Black delays developing his minor pieces until he knows where they will be best placed. Flexibility is the key to this opening: depending on White's choices, Black may develop his queens knight to c6 or d7 and his light-squared bishop to b7 or d7. Additionally, Black retains the option of playing ...e6 or ...e5, depending primarily on White's deployment of the bishops: 6.Bc4 and 6.Bg5 are both moves that directly control d5 and so ...e7-e5 in those cases cannot be recommended. It is, however, this flexibility that makes the Najdorf a hot favorite with the world's chess elite.

5...a6 is not without its own purposes, however. On a6 the pawn prevents a White piece from coming to b5. It also supports the advance of the b-pawn, ...b5. If Black achieves this thrust, he will gain space on the queenside, further restrict Whites pieces, and create the option to fianchetto the light-squared bishop with ...Bb7. White should be ready to counter a minority attack on the queenside. One idea, however, is to allow ...b7-b5, and then counter with a2-a4, hoping to prove that the pawn thrust has only weakened Black's queenside.

The most aggressive move for White is 6. Bg5, threatening to exchange the knight, giving Black doubled pawns. 6. Be2 is the classical main line, in which White hopes to impose his/her greater positional understanding. 6. Be3 is clearly the main line today, the English Attack, in which White has a simple plan of playing Qd2, castling queenside, playing f3 to secure e4 and g4 and pushing forward the kingside pawns and mating Black. There are also some deeper, more subtle positional ideas behind this move, but that is the basic outline of White's plan. Today, the variation with 6. Bc4 is considered unpromising for White, but was a Fischer favorite in the early 1960s and can lead to a brutal piece attack if Black is not careful. 6. f3 is mainly a transpositional tool for the English attack (to avoid the annoying line 6.Be3 Ng4). 6. f4 was favored by Tal and the Hungarian players Leko and Judit Polgar, hoping to directly pressure e5 and in some cases attack along the f-file. Fischer's other contribution to Najdorf theory, 6. h3, is rarely seen, preparing g2-g4 to fianchetto the bishop, while the immediate fianchetto, 6. g3, is out of favor now. 6. a4, preventing ...b7-b5, is considered innocuous, as it leaves a hole on b4 for Black's knight.